It is well known that in the reactor, alongside the conversion reaction of butane into maleic anhydride: EQU C.sub.4 H.sub.10 +3.5 O.sub.2 .fwdarw.C.sub.4 H.sub.2 O.sub.3 +4H.sub.2 O
other secondary reactions take place, among which the main ones are butane combustion reactions forming carbon monoxide and carbon dioxide: EQU C.sub.4 H.sub.10 +4.5 O.sub.2 .fwdarw.4CO+5H.sub.2 O EQU C.sub.4 H.sub.10 +6.5 O.sub.2 .fwdarw.4CO.sub.2 +5H.sub.2 O
Carbon monoxide is normally produced in excess and the molar ratio between carbon dioxide and carbon monoxide typically ranges between 0.6 and 1.0.
Reactions are strongly exothermic and the heat of reaction is suitably removed by circulating a coolant (usually molten salts) in the reaction section which thereafter release heat to a steam generator.
Conversion, selectivity and reaction yield depend on reaction conditions, mainly on feed composition, pressure, temperature and space velocity (the latter measured as standard volume of gas fed per hour per catalyst volume unit).
By conversion it is meant the butane percentage in weight fed to the reactor, which is transformed into the product or by-products.
By selectivity it is meant the amount of maleic anhydride expressed as the percentage (w/w) of the butane converted.
The conversion product by selectivity determines yield, which identifies the amount of maleic anhydride produced, expressed as total butane percentage in weight fed to the reactor.
Non-converted butane is present in the reaction effluent.
The maleic anhydride produced is recovered by selective absorption of maleic anhydride from reaction gases by means of an absorption medium which may be water or a selective organic solvent for instance preferably chosen among diesters of phthalic acid such as dibutyl phthalate and dioctyl phthalate.
Conventionally, absorption is carried out at a pressure which is slightly higher than atmospheric pressure, and which is sufficient to ensure exhaust gas transfer to an incinerator where organic compounds (mostly butane) are burnt and, after heat recovery, are exhausted into the atmosphere.
The result is that non-converted butane jeopardizes the process both in terms of yield, therefore higher raw material costs, and in terms of a higher carbon dioxide release into the air.
In order to increase yield, a fraction of the exhaust gases may be taken into consideration for recycling into the reaction.
Recycling of exhaust gases--containing non-converted raw materials--is a well-known procedure which has been used in various industrial processes, including catalytic oxidations in the vapor phase.
For example, this procedure is normally used in the catalytic oxidation of ethylene to ethylene oxide.
In the field of maleic anhydride production from butane, reaction gas recycling is detailed in the article "Oxidation of Butane to Maleic Anhydride", by Bissot and Benson, found at pages 57-60 of Industrial Engineering Chemistry, Book 2, no. 1, March 1963.
However, the article describes recycling within a process including a number of reactors connected in series with maleic anhydride separation between them.
This process received no industrial interest because of its complexity and the high investments that it involved.
Recycling was also reported in a number of patents (such as U.S. Pat. No. 3,899,516, U.S. Pat. No. 3,904,652, U.S. Pat. No. 4,222,945, U.S. Pat. No. 4,342,699; U.S. Pat. No. 5,011,945). All of them featuring use of oxygen or enriched air as oxidizing medium.
All patents, patent applications, and publications referred to in the present specification are incorporated herein by reference in their entirety.
In all processes using oxygen, exhaust gas recycling to the reaction is an essential factor, it being anyhow necessary to dilute oxygen and prevent explosion hazards.
Moreover, these processes are characterized by the operating conditions which remarkably differ from processes wherein air is used.
A typical use is high butane charge concentrations, obtaining low conversions per pass, so as to limit formation of gaseous by-product such as carbon monoxide and carbon dioxide which should be removed by releasing a fraction of exhaust gases.
U.S. Pat. No. 4,231,943 discloses exhaust gas recycling combined with use of air as oxidizing medium incorporated herein by reference in its entirety. The process described in this latter patent is inspired by principles which are typical of processes based on use of oxygen, i.e. low butane conversions per pass, relatively high concentration of butane and low concentration of oxygen in the feed. Process chemistry shows that, even under optimum conditions, when using air, at least 4 tons of inert gas (nitrogen) should be released per each ton of maleic anhydride produced.
Considering the high inlet and outlet butane concentrations with regard to the reactor, this operation involves very high butane losses in the released gases.
In order to prevent this, U.S. Pat. No. 4,231,943 provides a unit for the removal of butane from the released gases by absorption on activated carbon.
Butane absorption by activated carbon--due to the large gas deliveries to be treated at low pressure--is complicated and requires very high and costly amounts of absorption medium.
U.S. Pat. No. 5,011,945 describes a total recycle process wherein the oxidizing medium is oxygen mixed with exhaust reaction gases, to a large extent consisting of carbon monoxide and carbon dioxide, in molar proportion of at least 1 to 1, where the butane oxidation catalyst is a catalyst of the phosphorus vanadium mixed oxide (V.P.O.) type with addition of a co-metal comprising molybdenum.
In the mentioned process the recycling gas contains a high concentration of carbon monoxide, presenting inherent risks of loss of control and of deflagration in the butane oxidation reactor.
U.S. Pat. No. 5,688,970 describes a process where the oxidizing medium is a mixture of air and a fraction of reaction exhaust gases, characterized by the fact that the recovery of maleic anhydride (and consequently the recycle of exhaust gases) is carried out under pressure.
Compared to the conventional technology, the above mentioned process, makes it possible to get an improved selectivity and a higher yield (and consequently a lower butane consumption), a lower power consumption, and also a reduction in the carbon dioxide release to the atmosphere.
Although in principle the process described in said U.S. Pat. No. 5,688,970 might be applied to use enriched air or oxygen as oxidizing medium, the potential advantages of the use of oxygen is limited by the fact that an increase of the percentage of exhaust gases being recycled, consequent to the use of oxygen, increases the concentration of carbon monoxide in the feed to the reactor, with enhanced risk of deflagration.
US Pat. No. 5,126,463 describes a process where the oxidizing medium is pure oxygen mixed into exhaust gases of reaction, characterized by the fact that the carbon monoxide produced in the reaction is converted into carbon dioxide reacting over an oxidation catalyst (copper oxide-manganese oxide). The advantages of such process appear to be limited by the fact that the exhaust gases of reaction contain a high concentration of carbon dioxide, specified to be not less than 60% by volume, preferably about 80% by volume, which may adversely influence the activity and lifetime of conventional V.P.O. catalysts.
Furthermore, since the concentration of carbon monoxide in the recycling gases is low, a large volume of recycling gases, shown to be about 18 ton of gas per ton of maleic anhydride produced in Table II of U.S. Pat. No. 5,126,463, is to be processed in the catalytic converter of carbon monoxide.
The above mentioned features may have a significant impact on the performances and economics of the process described in the abovementioned patent.